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Date:   Sun, 28 Mar 2021 08:18:37 +0200
From:   Willy Tarreau <w@1wt.eu>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Mateusz Jonczyk <mat.jonczyk@o2.pl>, linux-kernel@vger.kernel.org
Subject: Re: Testers wanted: Atom netbooks with x86_64 disabled by BIOS
Message-ID: <20210328061837.GA22710@1wt.eu>
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 <20210327232551.GA20783@1wt.eu>
 <87lfa8cchf.ffs@nanos.tec.linutronix.de>
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Hi Thomas,

On Sun, Mar 28, 2021 at 03:07:24AM +0200, Thomas Gleixner wrote:
> On Sun, Mar 28 2021 at 00:25, Willy Tarreau wrote:
> > On Sat, Mar 27, 2021 at 10:13:22PM +0100, Mateusz Jonczyk wrote:
> > FWIW I tested on my ASUS 1025C which runs on an Atom N2600 forced to
> > 32-bit. I had already tried in the past but wanted to give it a try
> > again in case I'd have missed anything. Sadly it didn't work, I'm
> > still getting the "requires an x86-64 CPU" message.
> >
> > Given these machines were really cheap, I've always suspected that they
> > employ cheaper, low-grade CPUs, possibly having been subject to reduced
> > tests where x86_64-specific parts were not even verified and might be
> > defective. This may explain why they forcefully disable long mode there,
> > but that's just speculation.
> 
> There are some of these '32bit only' CPUs out there in the wild which
> actually support long mode. Some of them even do not have the long mode
> CPUID bit fused out.

Yes, I'm aware of this as well. We might even have talked to the same
"victim" :-)

> But whether it works is a different story:
> 
>   - If the CPUID bit is on, then the chance is high, but it runs out of
>     spec (guarantee wise)
> 
>   - If it's off is still might work by some definition of work as they
>     might have fused off more or there are actual defects in some 64bit
>     only area which are irrelevant when in 32bit mode.
> 
> Even if it could work perfectly fine, the BIOS/SMM/ucode can prevent
> switching to long mode.
> 
> It's a lost cause.

I agree. While I bought this netbook to have a 64-bit CPU and was extremely
disappointed, after seeing that it was not just a matter of "oops we forgot
to enable LM", I concluded that it was pointless to try to go further, as I
would never trust it anyway.

Cheers,
Willy